Just depends on how much you're willing to spend for your gaming capability. The GT 640 is not a step up from the 550 Ti in terms of performance (it's about 90% the speed), but it is an astonishingly capable card considering how little it costs. Considering ours is only a $69 card, I was amazed when I tested that Batman: AC was actually very playable in triple display 3840x720 with medium presets.

That being said, if you're looking for a card in this price range I would highly recommend Galaxy's GC edition GT 640 for reasons beyond just my being the Galaxy rep. The overclock is nice, but the main selling point on the GT 640 GC is the cooling, which is near undetectably silent. R&D measured 29dB under max load and my own sound meter couldn't pick it up at all.